1. Field of the Invention
This invention relates to the treatment of autodeposited coatings in order to retain longer or to enhance the corrosion resistance provided by the coatings on metallic surfaces, particularly ferriferrous articles, including zinc coated (also called "galvanized") steel.
2. Statement of Related Art
Autodeposition is a generic term used to describe the deposition of a substantially uniform organic binder containing film on a metal surface, generally preferably pre-cleaned, without the use of externally imposed electric current in the deposition process. Autodeposition involves the use of an aqueous coating composition containing dispersed organic resin, usually at relatively low solids concentration such as 5 to 12%, normally less than 10%, to form a coating of relatively high solids concentration, usually greater than 10%, on a metallic surface immersed therein, with the coating increasing in thickness and mass the longer the time that the metallic surface is immersed in the coating composition. Because the autodeposition process is driven chemically, rather than electrically, it coats wherever the solution wets the surface of the substrate, depositing a relatively uniform coating on even intricate or complex shapes or designs of the substrate.
In general, autodepositing compositions are aqueous acid solutions having solid resin particles dispersed therein in very finely divided form. The coating formed while the metal substrate used is immersed in the bath is generally wet and fairly weak, although sufficiently strong to maintain itself against gravity and moderate spraying forces. In this state the coating is described as "uncured". To make an autodeposition coated object suitable for normal practical use, the uncured coating is dried, usually with the aid of heat. The coating is then described as "cured".
Basic constituents of an autodepositing composition are water, resin solids dispersed in the aqueous medium of the composition, and activator, that is, an ingredient or ingredients which convert the composition into one which will form on a metallic surface a resinous coating which increases in thickness or a real density as long as the surface is immersed in the composition. Various types of activators or activating systems are known. The activating system generally comprises an acidic oxidizing system, for example: hydrogen peroxide and HF; HNO.sub.3 ; a ferric ion containing compound and HF; and other combinations of (i) soluble metal containing compounds such as, for example, silver fluoride, ferrous oxide, cupric sulfate, cobaltous nitrate, silver acetate, ferrous phosphate, chromium fluoride, cadmium fluoride, stannous fluoride, lead dioxide, and silver nitrate, in an amount between about 0.025 and about 50 grams per liter (hereinafter often abbreviated as "g/L"), with (ii) one or more acids such as hydrofluoric, sulfuric, hydrochloric, nitric, and phosphoric acids and organic acids such as, for example, acetic, chloroacetic, and trichloroacetic acids.
Autodeposition composition can be used to form coatings which have good aesthetic properties and which protect the underlying metallic substrate from being degraded, e.g., corroded by water. Many applications however require the autodeposited coating have particularly good properties for use. Various means have been developed to improve the properties of autodeposited coatings including:
(a) chemical pretreatment of the metallic surface prior to forming the coatings; PA1 (b) selection of specific particular resins which form the coating and PA1 (c) chemical post-treatment of the freshly formed or uncured coating. PA1 1 ) between 45 and 99%, based on the total weight of monomers used, of vinylidene chloride monomer; PA1 2) from about 0.5 to 30 weight percent based on the total weight of(1) and (2) of a second relatively more hydrophilic ethylenically unsaturated monomeric material wherein such monomeric material has a solubility in both the water phase and the oil phase of the polymer latex of at least 1 weight percent at the temperature of polymerization; and PA1 3) from about 0.1 to about 5 weight percent based on the total weight of other monomers of an ionic, significantly water-soluble material which is copolymerizable with (2) and is selected from the group of sulfonic acids and their salts having the formula: EQU R--Z--Q--(SO.sub.3).sup.- M.sup.+ PA1 wherein the moiety "R" is selected from the group consisting of vinyl and substituted vinyl, for example, alkyl-substituted vinyl; the symbol "Z" represents a difunctional linking group which will activate the double bond in the vinyl group; --Q-- is a divalent hydrocarbon moiety having its valence bonds on different carbon atoms; and the symbol "M.sup.+ " represents a cation. PA1 A) an aqueous concentrate comprising about 350 to about 550 g/l of resin particles, preferable the aforementioned vinylidene chloride-containing resin particles, and about 10 to about 550 g/l of pigment; and PA1 B) an aqueous concentrate prepared from about 0.4 to about 210 g/l of HF and a water soluble ferric-containing compound in an amount equivalent to about 1 to about 100 g/l of ferric iron.
U.S. Pat. No. 4,800,106, the entire disclosure of which, except to the extent that it may be inconsistent with any explicit statement herein, is hereby incorporated herein by reference, describes a number of references dealing with various treatments of uncured autodeposited coatings including the treatment of freshly formed autodeposited coatings with acidic, aqueous solution of one or more chromium compounds to improve corrosion resistance. While chemical treatments such as "reaction rinses" (which are defined as any rinses containing deliberately added ingredients other than water and air), following autodeposition coatings, can provide for improved corrosion resistance, gloss or other properties, oftentimes the treatment creates problems such as waste disposal problems. Thus the use of chromium containing compounds in post-treatment raises a disposal and environmental problem or disadvantage because the chromium must first be removed or otherwise treated before disposal to waste.
In the past, cold rolled steel and galvanized steel often required different autodeposited coatings, requiring different post-treatment, i.e. different reaction rinses for the coatings. One object of the present invention is to provide a process which includes a reaction rinse which will retain, or improve, the corrosion resistance properties of the autodeposited coating while employing non-chromium containing materials which are environmentally acceptable, raising no disposal problems.
A further object of the present invention is to provide a single reaction rinse for coatings on a variety of metal substrates, particularly on both cold rolled steel and galvanized steel. In this way a single reaction rinse may be used for composite objects that contain two or more distinct types of metal surface areas and autodeposited coatings thereon, so that simultaneous processing of such composite objects can be more efficiently carried out.